Embedded systems requiring high efficiency, high output current, and low production volume often use step down DC-DC converters, also known as buck converters. A buck converter generally employs Pulse Width Modulation (PWM) control, e.g., a PWM voltage-mode controller or a PWM current-mode controller.
Voltage-mode controllers use a Proportional Integral Derivative (PID)-type continuous transfer function, as compared to a triangle signal, to produce a modulated signal (the PWM signal). Because the PID-type function only uses the output voltage, the output filter forms a second order low pass filter with two poles. After adding an integral action to reduce steady state output error, the open-loop transfer function becomes a three-pole function. Whatever the designer does, however, the DC-DC converter is unstable without two correction zeroes. The compensation scheme required to provide such correction zeroes is highly sensitive to process variations and require a complex proper calibration system.
Current-mode controllers regulate the current supplied to a power inductor to regulate the output voltage. A current-mode controller operates using two loops: an internal current loop, which regulates the inductor current, and an outer voltage loop. Because the internal current loop forms a high bandwidth loop, the inductor may be modeled as a current source, such that the power-stage's transfer function is a first order function with a single pole defined by the output capacitor and the resistive load. The compensation required to stabilize the current-mode controller is much less complex than that required for the voltage-mode controller, and the overall performance is much better. However, current-mode controllers require measuring the inductor's current. Further, current-mode controllers may be unstable in some circumstances, e.g., when the required duty cycle is higher than 50% when the inductor's peak current is regulated, or when the required duty cycle is lower than 50% when the inductor's valley current is regulated. Current-mode controllers also have a tendency towards subharmonic oscillation, non-ideal loop responses, and an increased sensitivity to noise. Slope compensation, where a small slope is added to the measured inductor current, may be employed to overcome these difficulties.
Conventional slope compensation, however, typically increases the complexity and cost of the current-mode controller. For example, conventional slope compensation requires complex and sensitive measurement circuitry to measure the inductor current, which often requires a large biasing current. Also, commonly used instantaneous measurement circuits are not precise enough to be used in a regulated loop and do not provide a sufficient bandwidth for small duty cycles. Further, slope compensation typically results in a lower efficiency due to the required voltage drop for a direct current measurement. Also, some conventional slope-compensation circuits require a slope generator, e.g., the saw-tooth generator used for the PWM modulator, and a fast adder to add the inductor current measurement and the generated slope. When considering a 3.2 MHz switching regulator, which is a common switching frequency, the slope generator is not particularly more complex than one for voltage-mode control. The adder, however, must have a bandwidth much higher than the switching frequency, e.g., greater than ten times the switching frequency. In addition, all of the components required for this complex circuitry require a large silicon area. Thus, there remains a need for a stable current-mode controller employing less complex, but still accurate slope compensation.